As it is known by the person skilled in the art, various hydrometallurgical routes have been developed for extraction of nickel and cobalt contained in laterite ores. The objective of said routes is to solubilize the metallic species by using inorganic acids for heap leaching either in tanks under conditions of atmospheric pressure and at temperatures below the boiling point or in pressurized vessels, followed by neutralization (removal of Cu, Fe, Cr, and Al), and an optional solid-liquid separation prior to purification of the solution and final recovery of nickel and cobalt in the metallic form or as an intermediate product. The conventional hydrometallurgical route is shown in FIG. 10.
Selective recovery of the metal present in the leach effluent is an important stage in drawing up the economic evaluation. In the specific case of nickel and cobalt, they have very similar chemical properties, which facilitates operations for the simultaneous recovery of said metals by precipitation of either mixed sulfides (MSP) or mixed hydroxides (MHP), or by solvent extraction in hydrochloric, ammoniacal, or sulfuric medium, or by ion-exchange extraction using polymeric ion-exchange resins.
The ion-exchange technique using polymeric resins for selective nickel adsorption may be applied in two different ways: resin-in-solution and resin-in-pulp.
In the resin-in-solution option of operation, a solution with dissolved metals is brought in contact with the resin, and generally, adsorption is carried out on a fixed bed, such as for example, in adsorption columns.
In the resin-in-pulp option of operation, the ore pulp or any other pulp is brought in direct contact with the resin by means of an agitation system, so that adsorption of the metal occurs without the need for prior solid-liquid separation of the pulp. The resin is then separated from the pulp by screening.
Either of the two options can be adopted in flow sheets of nickel laterite ore processing. For resin-in-solution operation, solid-liquid separation is required, except in heap leaching cases where the resulting product is already in the form of solution. In this solid-liquid separation stage, besides its significant operational cost, there is some loss of nickel due to the inefficiency of the process, owing to the difficulty of washing the solids and recovering the dissolved species. In comparison, the resin-in-pulp process involves direct recovery of the dissolved metal from the leach pulp by means of an ion exchanger, thus eliminating the need for solid-liquid separation.
Although the application of resin-in-pulp has some advantages when compared with resin-in-solution, there are some limitations and technical risks in said application, such as the absence in the market of many resins having sufficiently high mechanical strength and abrasion resistance to withstand contact with the pulp. For this reason, the application of resin-in-solution is still often considered to be the best option.
Prior to application of either resin-in-solution or resin-in-pulp, there may be the need for acidity neutralization, pH elevation, and elimination of impurities through precipitation, as shown in FIG. 10, which illustrates the conventional process.
Current resins for selective recovery of nickel, that are commercially available at prices considered attractive, have two marked limitations:
1 Because of the high selectivity for H+ ions, the pH of the solution must be increased to values greater than pH=3, so that most of the resins become selective for nickel and present high adsorption performance for this metal. Otherwise, the excess H+ ions (low pH) will be preferably adsorbed to the detriment of the nickel adsorption process.
2 Every effluent solution from the acid leaching of nickel ores contains various dissolved metals regarded as impurities. Since every resin selective for nickel is also selective for iron, copper, and aluminum, even for lower concentrations of these elements (iron, copper and aluminum) in the solution, it is necessary to treat the solution beforehand, for elimination of these impurities.
The problems aforementioned are currently solved by the adoption of a neutralization stage in which lime, limestone, soda, or ammonium is added. Although this procedure overcomes restraints, it also has its drawbacks, such as significant losses of nickel, which is co-precipitated together with impurities, and, in the case of resin-in-column operation, the need for the onerous solid-liquid separation stage following neutralization.
As a solution for the abovementioned obstacles, mainly the neutralization stage, prior removal of impurities, pH elevation, and consequent losses of co-precipitated nickel, it is suggested herein that resins operating within low pH ranges may be used for example, those containing the functional group 2-picolylamine and concurrently utilizing the hybrid process technology, which eliminates the need for removal of impurities prior to the resin stage, as described in document BR 0600901-8. Thus, the effluent obtained from the leaching stage in the form of either pulp or solution is conveyed directly to the ion-exchange resin stage. The resin hybrid process, as described in the document BR 0600901-8, is necessarily and directly applicable to leach effluents in the form of solution (heap leaching) and in the form of pulp (tank leaching). This process comprises the following stages: processing (1) of the laterite ore (O), followed by either atmospheric or pressure leaching (2) and including the option of treatment of solution from solid-liquid separation in existing plants (2), with the process being characterized by the fact that it includes a circuit comprised of cationic or chelant resins, in which the first stage (3), with ion-exchange resins (Re), presents specific selectivity conditions for the removal of iron, copper, and aluminum and pH elevation, and the second stage (4), with ion-exchange resins (Re) allows recovery of the nickel and cobalt.
A number of documents address the various forms of use of resin applied to effluents from the leaching of nickel ores. Some documents deal with the use of resin applied to the effluent solution, while others deal with the use of resin applied to the effluent pulp. All methods mentioned in the state of the art section require, in some way or other, the adjustment of the pH either before or during contact with the ion-exchange resin.
A process for direct recovery of nickel and cobalt from certain ores is described in document U.S. Pat. No. 6,350,420. In one of the embodiments of the American invention, the nickel ore is leached with mineral acid so as to solubilize the metals, and consequently, form a pulp composed of a solution rich in these metals and leach residues. The leach effluents come in contact with an ion-exchange resin that selectively recovers the nickel and cobalt from the pulp. Preferably, the ion-exchange resin is added to the neutralized pulp. During the contact of the resin with the pulp, the pH is adjusted with the addition of a neutralizing agent. This is the greatest advantage of said American process, as pH control is performed in situ during ion-exchange extraction.
The use of resins applied to a leach solution after the leach effluent neutralization and solid-liquid separation stages is mentioned in documents AU 699127 and U.S. Pat. No. 5,571,308.
The new technological developments are aimed at creating increasingly less costly and more efficient processes. The known technologies, however, are often restricted to certain unit operations, and do not address some basic operational needs of other ore types with typical characteristics. With the new developments, some adaptations need be made in the existing prior flow sheets so as to make for technical improvements aimed at greater cost-effectiveness of the process. Additionally, the process itself may be improved and adapted to the new trends for higher efficiencies.
One other drawback of the conventional ion-exchange processes for recovery of nickel and cobalt from leach effluents is the fact that the neutralization stage is always necessary. Moreover, the solid-liquid separation stage is optional, and where it is not applicable the process is known as resin-in-pulp.
One other drawback of the conventional ion-exchange processes for recovery of nickel and cobalt from leach effluents is the fact that the flowsheets that had been developed often do not offer solutions for the technological challenges. Most times, the efficiency of unit operations is impaired, much as a result of a lack of optimization and development of new techniques for improvement of the existing ones.